Littérature scientifique sur le sujet « Nonlinear buffeting »

In order to eliminate buffeting problems caused by the traditional sliding mode control algorithm of induction motors, this paper adopts a continuous control term. In the meanwhile, a simple observer is designed to estimate the rotor flux and rotor torque. Besides, the mechanical equation of induction motor is applied to calculate the motor speed. Finally, according to the results of simulation, the buffeting problems are basically eliminated and thus the method mentioned in this paper is proved feasible.

To determine its actual dynamic responses under the wind loads, modal identification from the field tests was carried out for the Kao Ping Hsi cable-stayed bridge in southern Taiwan. The rational finite element model has been established for the bridge. With the refined finite element model, a nonlinear analysis in time domain is employed to determine the buffeting response of the bridge. Through validation of the results against those obtained by the frequency domain approach, it is confirmed that the time domain approach adopted herein is applicable for the buffeting analysis of cable-stayed bridges.

Technique of feedforward and feedback optimal vibration control and simulation for long-span cable-bridge coupled systems is developed. Buffeting loading systems of long-span cable-bridge structure are constructed by weighted amplitude wave superposition method. Nonlinear model of cable-bridge coupled vibration control system is established and the corresponding system of state space form is described. In order to reduce buffeting loading influence of the wind-induced vibration for the structure and improve the robust performance of the vibration control, based on semiactive vibration control devices and optimal control approach, a feedforward and feedback optimal vibration controller is designed, and an algorithm is presented for the vibration controller. Numerical simulation results are presented to illustrate the effectiveness of the proposed technique.

This paper presents the aerodynamic admittance functions (AAFs) of a double-deck truss girder (DDTG) under turbulent flows. The objective of the investigation is to identify AAFs using a segment model wind tunnel test. All of the wind tunnel tests were based on the force measurement method and conducted in a passive spire-generated turbulent flow. The segment model adopts a typical DDTG section and is tested in the service and construction stages under 0°, 3°, and 5° wind attack angles. Furthermore, a nonlinear expression is put forward to fit the identified AAFs. The buffeting responses of a long-span road-rail cable-stayed bridge are then calculated for both the service and construction stages using an equivalent ‘fish-bone’ finite element model of the DDTG. The unsteady effect of the buffeting force is considered based on quasi-steady buffeting theory using the identified AAFs. The calculated buffeting responses are finally compared with those for two other AAFs (AAF = 1.0 and the Sears function). The results indicate that the traditional AAFs overestimate vibrations in the vertical and torsional directions but underestimate vibrations in the lateral direction. The identified AAFs of the DDTG can be regarded as a reference for wind-resistant designs with similar girder sections.

Buffeting flow on transonic aerofoils serves as a model problem for the more complex three-dimensional flows responsible for aeroplane buffet. The origins of transonic aerofoil buffet are linked to a global instability, which leads to shock oscillations and dramatic lift fluctuations. The problem is analysed using the Reynolds-averaged Navier–Stokes equations, which for the foreseeable future are a necessary approximation to cover the high Reynolds numbers at which transonic buffet occurs. These equations have been shown to reproduce the key physics of transonic aerofoil flows. Results from global-stability analysis are shown to be in good agreement with experiments and numerical simulations. The stability boundary, as a function of the Mach number and angle of attack, consists of an upper and a lower branch – the lower branch shows features consistent with a supercritical bifurcation. The unstable modes provide insight into the basic character of buffeting flow at near-critical conditions and are consistent with fully nonlinear simulations. The results provide further evidence linking the transonic buffet onset to a global instability.

To determine its actual dynamic responses under the wind loads, modal identification from the field tests was carried out for the Kao Ping Hsi cable-stayed bridge in southern Taiwan. The dynamic characteristics of the bridge identified by a continuous wavelet transform algorithm are compared with those obtained by the finite element analysis. The finite element model was then modified and refined based on the field test results. The results obtained from the updated finite element model were shown to agree well with the field identified results for the first few modes in the vertical, transverse, and torsional directions. This has the indication that a rational finite element model has been established for the bridge. With the refined finite element model, a nonlinear analysis in time domain is employed to determine the buffeting response of the bridge. Through validation of the results against those obtained by the frequency domain approach, it is confirmed that the time domain approach adopted herein is applicable for the buffeting analysis of cable-stayed bridges.

In the last thirty years, the aerodynamic nonlinearities related to the slow variation of the angle of attack produced by large-scale atmospheric turbulence and their impact on the buffeting response of long-span suspension bridges have been a hot topic in wind engineering research. Self-excited forces accounting for such an effect of turbulence have been crucial in predicting the dynamic response of bridge sectional models and long-span suspension bridges subjected to multi-harmonic gusts and the turbulent wind, respectively. Despite several nonlinear aerodynamic models produced by the scientific community throughout the last years, only few studies on full suspension bridges nonlinear buffeting response in realistic turbulent flows are available. This doctoral work addresses this topic, aiming to enlarge the understanding of the effects of turbulence on the suspension bridge buffeting response. The first contribution of this work concerns nonlinear aerodynamic load modelling. Indeed, large-scale atmospheric turbulence produces large-amplitude low-frequency fluctuations of the angle of attack that can significantly change the self-excited and the external buffeting forces acting on a bridge deck. Assuming that the angle of attack varies slowly compared to the bridge motion, a time-variant linear model relying on Roger’s rational function approximation (RFA) of the force transfer function is proposed for modelling self-excited forces. In particular, an existing model is improved by a flexible fitting of the RFA directly in the multivariate space of reduced velocity and angle of attack. Another contribution of the present work is setting up an experimental procedure based on bi- or multi-harmonic forced-vibration tests to underscore the variation of magnitude and phase of self-excited forces under a time-variant angle of attack. These wind tunnel tests also allowed a sound experimental validation of the proposed model, considering two quite different bridge deck cross-sections as case studies. Aerodynamic derivatives for various angles of attack were measured to determine the model parameters. Despite its simplicity, the model yields accurate results up to relatively fast variations of the angle of attack, and it can reproduce the complicated behaviour of the self-excited forces revealed by the experiments. The model performance strongly depends on the goodness of the RFA-based fitting of aerodynamic derivatives, and the excellent results obtained were possible thanks to the high flexibility of the proposed method. Then, the nonlinear external buffeting forces are formulated to achieve a reasonable compromise between the conflicting needs of modelling both nonlinear and unsteady effects of wind velocity fluctuations. Then, the proposed 2D RFA model for self-excited forces and the nonlinear buffeting forces are incorporated into a stochastic time-variant state-space framework to assess the nonlinear buffeting response of a suspension bridge. The most important feature of this model is the modulation of the self-excited forces due to the spatio-temporal fluctuation of the angle of attack produced by low-frequency turbulence. Such an angle of attack accounts for the spatial wind correlation along the bridge girder. The model is applied to the Hardanger Bridge in Norway, considering different wind conditions. Indeed, the aerodynamic derivatives of this bridge deck cross-section present a strong dependence on the mean angle of attack. Moreover, a novel approach is suggested, diversifying the cut-off frequencies for the considered input motion components in the self-excited forces. In the wake of this, the thesis also investigates the sensitivity of the response statistics to the model cut-offs used to separate the low-frequency and the high-frequency turbulence band. The results emphasise the significant impact on the buffeting response and flutter stability of considering time-variant self-excited forces, though in specific cases the classical linear time-invariant approach is found to provide accurate predictions of the bridge vibrations.

<p>This paper presents a nonlinear framework to simulate the buffeting response of long-span bridges under Typhoon winds by using a Volterra series-based wind force model. First, the non-stationary wind fields are generated around the bridge using evolutionary power spectrum of the measured wind speeds. Subsequently, the nonlinear buffeting load model is formulated in time-domain by employing the Volterra series. Then, these Volterra kernels are identified from flutter derivatives. At last, the wind forces are applied to 3D fishbone finite element model of a suspension bridge and nonlinear buffeting analysis is performed. The time history analysis results show a good agreement in the simulation of Typhoon-induced buffeting response when compared with the measurement data of bridge displacements. Also, the analysis results are compared with the simulation results obtained from the existing wind load models to show the efficacy of the proposed framework.</p>

The dynamics of an aeroelastic system composed of a panel forced by unsteady buffeting aerodynamic loads is investigated numerically. The focus is on detecting parametric changes in the system. The upstream end point of the panel is considered supported by a spring of variable stiffness. Changes of in the stiffness of the spring are detected by exploring the chaotic dynamics of the panel.

<p>The cosine function and two modulation functions are separately selected to generate the time- varying mean wind and fluctuating wind speed, and their effects on nonlinear buffeting responses of a super long-span suspension bridge were investigated in this paper. Firstly, two non-stationary wind speeds models were validated by the classical power spectrum density, and could effectively simulate the non-stationary characteristics. Secondly, the time histories and RMS values of three displacement responses of the bridge deck under two non-stationary wind speeds with three different values of &#120574;&#120574; and &#120579;&#120579; were compared, respectively. Results show that the torsional and lateral displacement responses under the non-uniform modulation function are larger than those under the uniform modulation function. Moreover, the RMS values in three displacement responses of the deck gradually become larger with the increase of &#120574;&#120574; or the decrease of &#120579;&#120579;.</p>

VIBIC (Vibration of Beams with Intermittent Contacts) is a non-linear dynamics computer code developed and maintained by Atomic Energy of Canada Limited over the past 40 years in collaboration with universities. Its main application is the assessment of possible vibration damage to steam-generator and heat exchanger tubes. This assessment is done by performing simulations of the vibration response of beam-like structures to various flow-induced excitation mechanisms, such as turbulence buffeting, vortex shedding, and fluidelastic excitation. Fluidelastic excitation is potentially the most damaging flow-induced excitation mechanism. The fluidelastic effect has, until now, been incorporated in VIBIC using the frequency-based Connors model. To properly perform a time-domain simulation of fluidelastic-induced vibration, a time-domain fluidelastic force model is needed. In the present work, a time-domain formulation of the fluidelastic forces based on the quasi-steady model is implemented in VIBIC. The time delay inherent to the quasi-steady model is taken into account by using a delayed displacement in the expression of the fluidelastic forces. The resulting modal equations are delay differential equations that are solved using a continuous extension of the Runge-Kutta method. Both linear and nonlinear fluid force models are incorporated. The fluidelastic instability results predicted by the models are compared to known theoretical and experimental results for validation. The predictions of the models are in good agreement with those results. The results given by the nonlinear quasi-steady fluidelastic forces are found to be more realistic than those of the linear quasi-steady fluidelastic forces. A realistic simulation of the post-instability behaviour is made possible through the use of the nonlinear fluidelastic forces.